Recent developments in computer technology have enabled so-called "virtual reality" computer systems to be produced. These systems provide complete visual stimulation for the user usually by means of a helmet mounted display apparatus comprising a pair of miniature Visual Display Units (VDUs) arranged to project their images into respective ones of the user's eyes such that the image displayed is perceived stereoscopically thereby creating the illusion that the image is three-dimensional. These systems allow the user to interact with the displayed image by providing a means of monitoring movement of the user's head and, in some examples, movement of his eyes and other parts of his body. The information obtained by monitoring these movements allows the system to generate and present different images to the respective eyes of the user corresponding to changes in his line of sight. An example of such a system is given in British patent no. 2201069.
Current virtual reality systems present the user with a graphics display which is entirely computer generated. The computer graphics system generates the complete visual image for each of the VDUs in the helmet mounted display. In many applications highly complex images need to be generated which, combined with the necessity to have them produced in real time, makes very heavy demands on computing power in terms of memory size and speed of access. One particularly notable problem is that of realistically representing the user's hands, legs and feet whenever the user's current line of sight would include those parts of his body. The result is that the systems are extremely expensive or they provide simpler images than are desirable for optimum realism.
Attempts have been made to overcome the need for high computing power by laying the computer generated image over the real image of a scene by using an optical combiner to combine the two images. Although these systems are less demanding in terms of what is required of the computer generated images because the images may then occupy a smaller area, the system now must determine which part of the scene viewed by the user the computer generated image should occupy as his line of sight changes as a result of eye and head movements. Current head position sensors are limited in terms of positional accuracy, and, when used with such visually coupled systems, any errors in the calculation of head position cause the computer generated image to move relatively and unrealistically within the real scene. Under certain circumstances this image motion can be extremely objectionable and may even cause nausea.